Field effect transistor device with contact in groove

ABSTRACT

A high power FET device includes a plated heat sink, a rear surface electrode disposed between a substrate and the heat sink, a via-hole extending through the substrate and containing a metal plating for electrically connecting the rear surface electrode and an element, such as the source electrode, of the FET device. A metallic layer extending from the rear surface to the front surface of the device protects the side walls of the substrate during handling. The side wall protection layer extends onto portions of the front surface of the substrate as a measurement electrode. The arrangement gives access to the source, drain, and gate electrodes of the device from the front surface for measuring the electrical characteristics of the device while it is still part of a wafer containing a large number of devices. Each device includes a separation groove outwardly spaced from the device and containing a metallic layer which becomes the side wall protection layer after dicing. Preferably, the separation grooves are wider and deeper than the via-holes.

This application is a continuation of application Ser. No. 07/370,249,filed Jun. 22, 1989, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a field effect transistor device and aproduction method therefor and particularly to a high power field effecttransistor device including via-hole electrodes.

BACKGROUND OF THE INVENTION

FIG. 4 shows a prior art high power field effect transistor (FET)device 1. The terms FET and FET device as used here respectively referto a field effect transistor have single source, gate, and drainelectrode elements and to an integrated structure incorporating acomplex field effect transistor structure, i.e., a structure having morethan one gate, source, and drain electrode element. Device 1 includesseveral interconnected field effect transistor electrode elements, i.e.,two drain fingers, four gate fingers, and three source elements that arerespectively electrically connected to common drain, gate, and sourceelectrodes. While FIG. 4 shows a single FET, multiple, interconnectedFET structures, each FET having the general arrangement of FIG. 4, maybe constructed on a single substrate. An example of such a multiple,interconnected FET structure employing five FETs disposed on a singlesubstrate and electrically connected in parallel is disclosed by Saitoet al in "X and Ku Band High Efficiency Power GaAs FETs" 1983 IEEE MTT-SDigest, pages 265-267.

The FET device 1 includes a gallium arsenide (GaAs) substrate 2, anactive region 3 disposed at a front surface of the GaAs substrate 2, anda gate electrode 5 having four gate fingers disposed on the frontsurface of the GaAs substrate 2. A drain electrode 4 disposed on thefront surface of the substrate 2 includes two drain fingers. The sourceelectrode 6 includes four source elements. Each of the gate fingers isdisposed between one of the source electrode elements and one of thedrain fingers. In the structure of FIG. 4, a single drain finger ofdrain electrode 4 serves two FET elements since a gate finger and asource electrode element lie on each of the two opposite sides of eachdrain finger of drain electrode 4.

Via-holes 9 extend through the GaAs substrate 2 from the front surfaceto the rear surface adjacent the source electrode elements. A via-holeelectrode 10 is disposed in each of the via-holes 9. An electrode 7disposed at the rear surface of substrate 2 is in electricalcommunication with the source electrode 6 through the via-holeelectrodes 10. The substrate 2 is mounted on and is in electrical andthermal communication with a plated heat sink (PHS) 8 that is formed byelectrolytic plating using the rear surface electrode 7 as a platingelectrode.

FIG. 9 shows a wafer 100 on which a plurality of FET devices 1 have beenformed. Wafer 100 forms the substrate of each device. After the deviceson the wafer 100 are completed, the wafer 100 is divided into theindividual FET devices 1 by severing, fracturing, etching, or the like.

FIGS. 5(a)-5(e) are cross-sectional views of the FET device 1 takenalong line V--V of FIG. 4. FIGS. 5(a)-5(e) illustrate the steps in aprocess for making the single FET device 1. Saito et al have not fullydisclosed the process steps they employ in making their multiple,interconnected FET. Therefore, the process steps illustrated in FIGS.5(a)-5(e) may not be the same as those employed by Saito et al. Thesteps illustrated in FIGS. 5(a)-5(e) are somewhat similar to thoseillustrated in Japanese Published Patent Application 62-122279. Usually,a plurality of FET devices 1 are actually manufactured on a single wafer100. However, only one device is shown in FIGS. 5(a)-5(e) forsimplicity. Reference to substrate 2 means wafer 100 in the processdescribed below until the step in which the wafer 100 is divided intoindividual FET devices 1.

As illustrated in FIG. 5(a), silicon is implanted in a portion of thefront surface of the p-type or semi-insulating GaAs wafer 100.Generally, that substrate has a thickness of about 600 microns and ionsare implanted to produce a dopant concentration of about 3×10¹⁷ /cm³near the front surface of the wafer. With the usual background dopinglevel of the wafer 100, the resulting n-type active region 3 has a depthof about 0.4 micron. The drain electrode 4, the source electrode 6, andthe gate electrode 5, including their respective fingers, are deposited,defined, and arranged on the active region 3 as shown in FIG. 5(a). Thedrain and source electrodes 4 and 6 form ohmic contacts with the wafer100. Those contacts may be made from an alloy of gold and germanium.Aluminum, titanium, or platinum is deposited as the gate electrode 5 andforms a Schottky barrier with the n-type region 3. The arrangement ofthe electrodes relative to each other is achieved with conventionalmetal deposition and photolithographic techniques.

A mask, such as a photoresist mask, is applied to the front surface ofthe wafer 100 and formed into a pattern to define the areas of via-holes9 adjacent the source electrodes 6. The via-holes 9, typically having adepth and width of about 30 microns, are formed by etching. Gold isselectively plated on the internal walls of the via-holes 9 and adjacentthe periphery of the via-holes 9 to a thickness of about 3 microns. Thedeposited gold produces via-hole electrodes 10 that are electricallyconnected to the source electrode elements 6 as shown in FIG. 5(b).

The wafer 100 that forms the GaAs substrate 2 of each of the devices 1is mounted at its front surface to a glass plate 22 with, for example,wax 21 or another adhering material. The wafer 100 is ground or lappedon its rear surface until its thickness is reduced to about 60 microns.Subsequently, through a combination of mechanical and chemicalpolishing, the substrate is made still thinner. The rear surface of thewafer 100 may be mechanically and chemically polished simultaneously byrubbing it against a cloth to which an etching solution is periodicallyapplied, for example, by dripping a liquid etchant onto the cloth.Eventually, the via-hole electrodes 10 are exposed at the rear surfaceof the substrate, as shown in FIG. 5(c), when the thickness of thesubstrate is reduced to about 25 to 30 microns. Similarly, Saito et alstate that in the preparation of their device including fiveinterconnected FETs, the substrate is chemically etched from the bottomside to reach the source pads. Likewise, the substrates in the processesillustrated in Japanese Published Patent Document 62-122279 are etchedfrom their rear surfaces to reduce their thicknesses.

After the electrodes 10 are exposed, a metallic electrode 7 is depositedon the rear surface of wafer 100 in mechanical and electrical contactwith electrodes 10. The electrode 7 may have three successivelydeposited metallic layers, such as titanium, gold, and titanium. A mask11 of a photoresist is deposited on electrode 7 and formed into apattern defining separation portions on electrode 7 for later separationof wafer 100 into individual FET devices 1. The resist is removed fromthe areas corresponding to the FET devices 1, thereby exposing theelectrode 7 in the areas of those FET devices. The outermost titaniumlayer is removed selectively, exposing the underlying gold film for asubsequent plating step. A metal, usually gold, is deposited by anelectrolytic plating process on the exposed portions of electrode 7 to athickness of about 60 microns, thereby forming the PHS 8 at the rearsurface of the wafer 100, as shown in FIG. 5(d).

In the Saito et al device the chip bottom is plated to form the PHS andthe side walls of the chip are plated simultaneously. Saito et al do notdisclose whether several of their devices are manufactured on a singlewafer simultaneously or whether single devices are prepared on discretechips. Whatever processing Saito et al use gives access to both the chipside wall and the PHS area so that they can be plated simultaneously.(The process of FIG. 5(d) does not provide that simultaneous dualaccess.) Thus, Saito et al form a protective side wall plated metal(gold) layer around the side wall of the substrate 2 of FIGS. 4 and 5that is not shown in those figures.

Returning to FIG. 5, the remaining portions of mask 11 and the portionsof the rear surface electrode 7 and of the GaAs wafer 100 that are notmasked by PHS 8 are successively etched and removed. These etching andremoval steps divide the wafer 100 into a plurality of FET devices 1,each with its substrate 2, as indicated in FIG. 5(e). The separated FETdevices 1 are released from the glass plate 22 by dissolving the wax orother resin 21. Then the FET devices are tested. Frequently the devicesare designed for and are operated at microwave frequencies. Theelectrical characteristics of those FET devices are measured with aspecial high frequency, low VSWR jig in which electrode 7 is contactedat the rear surface of the substrate 2, and the drain 4 and gate 5 arecontacted at the front surface. Those devices that function properly areemployed and the others are discarded. The handling of the devices 1 inorder to carry out testing exposes them to mechanical damage. It wouldbe preferable to test the devices before separating them from the wafer100 to reduce damage, thereby increasing yields, and to save time.

In the FET devices described with respect to FIG. 5, the lateralsurfaces of the GaAs substrate 2, i.e, the side wall surfaces joiningthe front and rear surfaces, of the individual devices are exposed.During packaging and testing, the devices are repeatedly handled. Thesubstrate 2 may crack during this handling when tweezers are employed tograsp the devices 1. Moreover, during die bonding employing automatedbonding equipment, a vacuum chuck or tweezers for engaging a device maycome into direct contact with the GaAs substrate 2, severely damagingit. The damage from handling reduces overall yields and increases thecost of the acceptable devices. It would, therefore, be desirable toprovide protection for the lateral surface of the substrate. In the FETdevices described by Saito et al, those side walls are protected by aplated gold layer.

In order to achieve high production yields with the prior art productionprocess described, the resist mask 11 has to be precisely aligned withthe electrode pattern. After its use, mask 11 must be completelyremoved. If the patterns of the resist mask 11 and the electrodes on thesubstrate 2 are misaligned, the PHS 8 will not be properly positioned,adversely affecting the FET device characteristics. Furthermore, asshown in FIG. 8, faults can occur in the edge of the resist mask 11. Asshown in FIG. 6, these edge faults can produce irregular lateral surfaceportions C in the PHS 8, the rear surface electrode 7, and the GaAssubstrate 2. An FET device 1 with such lateral surface variations cannotbe mounted on a carrier having a precisely controlled size.

In processing to prepare high power FETs having a PHS, a mask alignmentstep between a pattern at the front surface and one at the rear surfaceis usually required. In FIG. 5(d) for example, mask 11 has to be alignedrelative to the locations of via-holes 9. While the alignment tolerancesrequired are not unreasonably difficult to achieve, alignment of maskson opposite sides of a wafer presents difficult problems. Usually, thealignment is carried out with the aid of infrared light that is shone onone side, passes through the substrate and via-holes, and is detected atthe opposite side. That infrared alignment step must be completed beforeany metallizations that can block the transmission of infrared light aredeposited on the wafer. Thus, it would be desirable to employ a processfor making high power FETs like those of FIG. 4 that avoids thenecessity of aligning masks on opposite sides of a wafer.

The external configuration, size, and clearances of the PHS 8 areprincipally determined by the thickness of the plated gold layer of thePHS 8 and the planarity of its plated side walls. The GaAs substrate 2itself is defined by etching using the PHS 8 as a mask. Therefore, inorder to achieve the desired dimensional precision of the substrate, theconfiguration of the plated layer must be precisely controlled so thatno irregularities occur in the patterning of the resist mask 11 and theetching of the rear surface electrode 7. In addition, no unwantedresidual portions of the mask 11 and electrode 7 must be left in placeafter the etching. However, it is difficult to remove the resist mask 11completely and to obtain a smooth lateral surface along the rear surfaceelectrode 7.

The manner of etching of the resist mask 11 is illustrated in FIGS. 7(a)and 7(b) which show enlarged views of the region within the circle Dfrom FIG. 5(d). As the plated PHS 8 increases in thickness, it extendsonto the resist mask 11 as shown in FIG. 7(a). In the example described,the PHS 8 extends across about 45 microns of the 3 micron thick resistmask 11. It is quite difficult to completely remove the resist 11 thathas been covered by the PHS 8, especially at the innermost portionsunder PHS 8. As shown in FIG. 7(b), some residual part of the resist 11may not be removed. When the rear surface electrode 7 and the GaAssubstrate 2 are etched to separate the wafer into the FET devices 1, theresidual resist, indicated as 11a in FIG. 7(b), is not etched, resultingin variations in the lateral surfaces of the substrate 2 as shown inFIG. 6 as portions B. Accordingly, it would be desirable to eliminatethe sequential masking, plating, and etching steps at the rear surfaceof the FET device 1.

SUMMARY OF THE INVENTION

An object of the present invention is to provide semiconductor devicesthat may be fabricated in large numbers on a wafer, that may be easilyseparated from each other and the wafer without damaging the devices,and that may be easily tested from the front surface before separationfrom the wafer.

An additional object of the invention is to provide an FET device thatmay be readily handled without damage and that has precise outsidedimensions.

Another object of the present invention is to provide a method forproducing such an FET device, particularly a method that avoids anyfront to rear surface mask alignment steps.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter. It should beunderstood, however, that the detailed description and specificembodiment are given only by way of illustration since variousmodifications and additions within the spirit and scope of the inventionwill be apparent to those of skill in the art.

According to one aspect of the invention, a large number of FET devicesare fabricated on a single wafer that acts as a common substrate for thedevices. The devices each include at least one via-hole extending intothe wafer from the front surface for connecting one of the electrodes,usually the source electrode, to an electrode disposed on the rearsurface of the device substrates. At least one circumferentialseparation groove extending into the wafer from the front surface towardthe rear surface and lying outwardly from an FET device is used forseparating the wafer into a plurality of devices. A metallic electrode,preferably gold, is disposed in the via-hole and in the separationgroove of each device.

When the wafer is formed into dice along the separation grooves, themetallic electrodes provide a side wall protection layer at the sidesurface of each of the FET devices to protect them during subsequenthandling. Preferably, the separation grooves are wider and deeper thanthe via-holes for easier separation of the devices from the wafer by achemical and/or mechanical means. The side wall protection layerpreferably extends onto the front surface of the individual devices topermit measurement of the electrical characteristics of the devicesbefore the wafer is separated into individual dice. A plated heat sinkis disposed at the rear surface of the wafer in contact with anelectrode for dissipating heat and increasing the power handlingcapacity of the device.

Each individual FET device according to the invention includes asubstrate having at least one via-hole extending through it and ametallic layer disposed in the via-hole connecting a source electrode onthe front surface of the substrate with an electrode disposed on therear surface of the substrate. That rear surface electrode has disposedon it a plated heat sink for dissipating heat generated during operationof the device. The side walls of the substrate lying between the frontand rear surfaces are covered with a metallic protection layer,preferably gold, that has been commonly deposited with the metalliclayer in the via-holes. The gate and drain electrodes are disposed onthe front surface of the FET device along with measurement electrodesextending from the side wall protection layer. All of the electrodes ofthe FET device are available at the front surface to permit testing ofthe electrical characteristics of the devices before the separation ofthe wafer into individual FET devices.

FET devices according to the invention are formed in a substrate at anactive region of the substrate. The electrodes of the device aredeposited on a front surface of the substrate in a predeterminedrelationship to each other. At least one via-hole is etched through thesubstrate from the front surface. A circumferential separation groovespaced from the active region of each device is etched from the frontsurface and is preferably wider and deeper than the via-hole. A metalliclayer is deposited in the via-hole and separation groove from the frontsurface of the substrate. The thickness of the substrate is reduced fromthe rear surface, preferably in a combined chemical and mechanicalprocess. A second metal layer is deposited on the rear surface of thesubstrate in contact with the metallic layer in the via-hole. A heatsink is plated on the rear surface electrode and each device isseparated from its neighbors on a wafer along the separation grooves.The separation may take place by etching, mechanical severing, or acombination of those processes. The metallic layer deposited in theseparation grooves protects the side walls of the substrate duringhandling. That protection layer may extend onto the front surface of thesubstrate to provide measurement electrodes so that all electrodes of anFET device are accessible from the front surface of the substrate fortesting before separation of the devices from the wafer. The metalliclayers and plated heat sink are preferably electrolytically depositedgold.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a perspective view of the structure of an FET device accordingto an embodiment of the present invention.

FIGS. 2(a)-2(e) are cross-sectional diagrams illustrating a productionprocess for the FET device of FIG. 1.

FIG. 3 is a diagram showing a plurality of FET devices on a single waferaccording to an embodiment of the invention.

FIG. 4 is a perspective view showing the structure of a prior art FETdevice.

FIGS. 5(a)-5(e) are cross-sectional diagrams illustrating a productionprocess for the FET device of FIG. 4.

FIG. 6 is an enlarged view of a portion of FIG. 4 illustrating problemsin the prior art production method.

FIGS. 7(a) and 7(b) are cross-sectional diagrams illustrating removal ofa resist mask in the prior art production method.

FIG. 8 is a diagram showing an edge fault in a resist mask.

FIG. 9 is a diagram showing a plurality of prior art FET devices on asingle wafer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a perspective view of a high power FET device 101 according toan embodiment of the invention. (Like elements of FET device 1 of FIG. 4and FET device 101 of FIG. 1 are given the same reference numbers.) FETdevice 101 includes an active region 3, a drain electrode 4 having twodrain fingers, a gate electrode 5 including four gate fingers, and threesource electrodes 6. A measurement electrode 7b on the front surface ofthe GaAs substrate 2, an electrode 7 at the rear surface, the sidesurface protection layer 7a, and the PHS 8 are all gold that is formedby plating. The via-hole electrodes 10 disposed on the internal surfacesof the via-holes 9 extend through the substrate 2 from the front surfaceto the rear surface. The electrode 7 disposed at the rear surface of theGaAs substrate 2 is connected with the source electrode 6 through theplated via-hole electrodes 10.

As shown in FIG. 1, FET device 101 includes a gold protection layer 7acovering the lateral or side surfaces of the GaAs substrate 2 betweenthe front and rear surfaces of the substrate. Measurement electrodes 7bare disposed on the front surface of the GaAs substrate 2 in electricalcommunication with the rear surface electrode 7 and source electrodes 6through the protection layer 7a. Measurement electrodes 7b, along withgate and drain electrodes 4 and 5, enable testing of the electricalcharacteristics of FET device 101 from its front surface.

The FET device 101 may be produced by the process steps illustrated inFIGS. 2(a)-2(e). In FIGS. 2(a)-2(e), a single FET device 101 is shown incross-section. In general, as illustrated in FIG. 3, a plurality of FETdevices 101 are formed in a matrix arrangement on a wafer 100'. Thus,the FET device 101 of FIGS. 2(a)-2(d) includes eight contiguouslyattached, but unillustrated, FET devices 101 all having wafer 100' as acommon substrate 2. The terms wafer 100' and substrate 2 are generallyinterchangeable until the wafer id divided into individual dice, eachcontaining an FET device 101. In FIG. 2(a), an active region 3 is formedin a portion of the front surface of the substrate 2 by ionimplantation. A drain electrode 4 having two drain fingers, a gateelectrode 5 having four gate fingers, and four source electrode elements6 are deposited and formed in the desired arrangement on the frontsurface of the GaAs substrate 2 in the same manner as described withrespect to FIG. 5(a).

Via-holes 9 are formed in the wafer 100' from the front surface to adepth of about 30 microns by an etching technique. Circumferentialseparation grooves 13 for separating the FET devices 101 from wafer 100'are also formed in the GaAs wafer 100' extending from the front surfacetoward the rear surface. These separation grooves 13 are referred to ascircumferential because each surrounds a respective FET device 101. Eachgroove 13 in plan view may have any shape that is spaced from the activeregion and electrodes of the respective surrounded FET device 101. Inthe embodiment of FIG. 3, the grooves are rectangular in plan view sothat the resulting FET devices 101 are rectangular. In the embodiment ofFIG. 3, the separation grooves 13 for different FET devices 101 arespaced from each other, but adjacent devices could employ commonseparation grooves.

Grooves 13 may be formed either at the same time as via-holes 9 areformed or, at least partially, thereafter. The separation grooves 13 arepreferably wider and deeper than via-holes 9. The formation of both thevia-holes 9 and the separation grooves 13 may be carried out at the sametime employing a dry etching technique, such as reactive ion etching. Inthat case, grooves 13 are etched longer than via-holes 9 are etched. Ifa wet, chemical etching technique is used, the via-holes 9 and theseparation grooves 13 may be etched simultaneously. Since the separationgrooves 13 are wider than the via-holes 9, the widths being defined by amask, such as a photo-resist mask, the separation grooves 13 are etcheddeeper by a single chemical etching step than are the via-holes 9. If arelatively large difference in the depths of the via-holes 9 and theseparation grooves 13 is desired, a two-step etching process is likelyto be required regardless of the etching technique employed. A dryetching process may produce via-holes and grooves having nearly square,i.e., right angle, internal and external corners, as shown in FIG. 2(b).Chemical etching is more likely to produce rounded corners in thegrooves instead of sharply angled corners. Rounded corners on FETdevices 101, especially at the edges of substrate 2, may provide betterprotection against cracking during handling. However, dry etching ispreferred over wet etching because it permits better control of the areaetched and the depth of the etching.

After the formation of the via-holes 9 and the separation grooves 13,the via-hole electrodes 10 are deposited on the internal surfaces of thevia-holes 9 and of the separation grooves 13 as shown in FIG. 2(b).Electrodes 10 are preferably plated gold and have a thickness of about 3microns. The source, drain, and gate electrodes and their respectivefinger elements may be protected by a photoresist mask during theplating of electrodes 10. In that case, subsequent removal of theprotective mask lifts off any metal plating that builds up on maskedareas on the front surface of the wafer 100'. Generally, the areasadjacent the via-holes and grooves are plated as shown in FIG. 2(b).

The front surface of the GaAs wafer 100' is mounted on a glass plate 22with wax 21 or another removable adhering material as in the prior artmethod. The thickness of the wafer 100' is reduced by polishing the rearsurface of the substrate until the electrodes 10 are exposed.Preferably, a combined mechanical and chemical polishing technique isemployed, i.e., mechanical rubbing combined with the application of arelatively slow etchant to produce the rear surface configuration ofFIG. 2(c). Generally, a non-planar rear surface is produced on wafer100' when the separation grooves 13 are deeper than the via-holes 9. Theplating of electrodes 10 is not attacked by the etchant and the etchingrate adjacent the separation grooves 13 is reduced when the electrodes10 are exposed. However, etching continues at the same rate elsewhere.The resulting wafer configuration includes a thicker substrate portionadjacent the separation grooves 13 than adjacent the via-holes 9. Theelectrodes 10 in the via-holes 9 are exposed when the thickness of thesubstrate 2 is reduced to about 25 to 30 microns.

Successively deposited layers of titanium and gold or nickel and goldare deposited on the rear surface of the reduced thickness wafer 100' bya vacuum deposition technique or an electroless plating method. Atemporary third layer, e.g., titanium, may be employed to protect thegold layer until just before plating of the PHS 8. The temporary layeris removed before PHS plating begins. The deposited metal layers producethe rear surface electrode 7. A gold PHS is deposited, for example, byelectrolytic plating, on the electrode 7 to a thickness of about 60microns as shown in FIG. 2(d).

Wafer 100' is detached from glass plate 22 by dissolving the adheringmaterial 21. The PHS 8 is fractured or cut, for example, by a dicer,along the separation grooves 13 to separate the FET devices 101 from thewafer 100' as indicated in FIG. 2(e). Since electrode 10 was depositedin separation grooves 13, the lateral surfaces of the separated FETdevices 101 are plated with metal, i.e., electrode 10. That plating,preferably gold, is the protection layer 7a of FIG. 1. Measurementelectrodes 7b are formed in the same plating step that producesprotection layer 7a. During the masking for the electrode layer 10, asshown in FIG. 2(b), additional portions are provided on the frontsurface of the wafer 100' so that measurement electrodes 7b are formed.Measurement electrodes 7b are produced either by masking the goldplating to protect it during etching or by appropriately masking thesubstrate during plating when the lift-off method is employed.

Since the lateral walls of the GaAs substrate 2 are covered with a metalprotection film 7a, the weak GaAs substrate 2 is protected frommechanical damage. Even when the substrate is picked up by tweezers, thetweezers do not directly contact the GaAs substrate 2, greatly improvingthe ability to handle the FET device 101 without damage. A still furtherimprovement in resistance to mechanical damage is provided whenprotection film 7a is concave, i.e., when wet etching is employed toform the grooves 13 rather than dry etching. The curved surface spreadsthe forces applied by the tweezers, reducing the pressure applied to thefragile substrate.

The measurement electrodes 7b on the front surface of the substratepermit the source, drain, and gate electrodes all to be accessed fromthe front side of the wafer 100'. Therefore, the electricalcharacteristics of each FET device 101 can be measured from the frontside of the wafer 100' during the production process, i.e., before thewafer 100' is divided into individual devices. The front surface accessmeans a high frequency jig or connector can be temporarily applied towafer 100' for the evaluation of each FET device 101. This arrangementprovides a significant improvement over FET devices 1 which cannot beevaluated without connecting a jig to both the front and rear surfaces.Thus, FET devices 1 could not be tested until wafer 100 was cut intodice.

Preferably the FET devices 101 are spaced apart from each other on thewafer 100'. The division of the wafer 100' into FET devices 101 ispreferably accomplished by etching from the front surface of the waferin the areas between devices 101, i.e., between the spaced apartseparation grooves 13. Thereafter, PHS 8 is cut at the rear surface. TheFET devices 101 are finally fully separated from each other, forexample, with a conventional dicing apparatus. Severing preferably takesplace along the separation grooves 13, producing the device shown inFIG. 2(e) with metal plating on its side surfaces.

The process for separating wafer 100' into individual devices is basedsolely on the pattern on the front surface of the substrate 2. Thus,there is no need to apply a separation pattern on the rear side of thesubstrate. In some previously known processes, individual separationpatterns are required for opposite sides of the substrate. In order toachieve the desired separation, those patterns must be aligned or theresulting devices may be defectively formed. As discussed in thebackground section, alignment of individual patterns on the front andrear surfaces of a wafer is a cumbersome process requiring use ofinfrared light and adjustment of the sequence of the process steps sothat metallizations that could interfere with the infrared light are notdeposited until after the alignment step is completed. However, in theinvention a front-to-back surface mask alignment is unnecessary. Theavoidance of such a pattern alignment step means that the wafer 100' canbe separated into FET devices 101 with a high degree of dimensionalprecision.

Since the via-holes 9 and the separation grooves 13 in the FET device101 are produced at the same time from the same pattern, there are nomisalignment errors between the holes and grooves that can arise whenthose features are separately defined in different patterning steps. Theplating 10 in the separation grooves 13, which are preferably deeperthan the via-holes 9, can be used as a target at the rear surface todetect the conclusion of the first portion of the polishing process inwhich the thickness of the GaAs wafer 100' is reduced. The polishing canbe temporarily stopped when electrode 10 in the bottom of the separationgroove is initially detected. Thereafter, polishing may be resumed untilelectrode 10 is exposed at the bottom of the via-holes 9. The detectionof the exposed electrodes 10 in via-holes 9 to indicate an etchingstopping point eliminates any need to monitor the thickness of the wafer100' during the polishing process.

Although the invention has been described with respect to certainpreferred embodiments, many variations are possible. Separation grooves13 can be formed with dicing apparatus or by a separate etching step.The depths of the via holes 9 and the separation grooves 13 can be madeequal to each other. Furthermore, the via-holes 9 and the separationgrooves 13 may be separately metallized. Although the invention has beendescribed with respect to an embodiment of an FET device having avia-hole electrode structure, the invention may also be applied to othercircuit elements and to integrated circuits incorporating via-holeelectrode structures.

We claim:
 1. A semiconductor device assembly comprising:a semiconductor wafer having opposed front and rear surfaces, said wafer being a common substrate; a plurality of substantially identical semiconductor devices formed on said wafer, each device including at least one via-hole extending into said wafer from said front surface toward said rear surface; at least one circumferential separation groove extending into said wafer from said front surface toward said rear surface farther than said at least one via-hole and lying outwardly from a first of said semiconductor devices for separation of said first semiconductor device from said wafer; a first metallic electrode disposed in said at least one via-hole for electrically connecting said respective semiconductor devices at said front surface; and a metal layer disposed in said at least one separation groove for forming a metal protection layer on surfaces of said semiconductor devices when they are separated from said wafer.
 2. The semiconductor device assembly of claim 1 wherein said at least one separation groove is wider than said at least one via-hole.
 3. The semiconductor device assembly of claim 1 wherein said first metallic electrode and said metal layer are gold.
 4. The semiconductor device assembly of claim 1 wherein said metal layer extends over portions of the front surface of said first semiconductor device from said at least one separation groove to form a measurement electrode for measuring electrical characteristics of said first semiconductor device before its separation from said wafer.
 5. The semiconductor device assembly of claim 4 including a second metallic electrode disposed on the rear surface of said substrate in electrical communication with said first metallic electrode at said at least one via-hole and with said metal layer.
 6. The semiconductor device assembly of claim 5 including a plated heat sink disposed on said second metallic electrode.
 7. The semiconductor device assembly of claim 1 wherein said semiconductor devices are field effect transistors, each including an active region in said wafer at said front surface and a source electrode, a drain electrode, and a gate electrode disposed on said front surface at said active region.
 8. The semiconductor device assembly of claim 7 wherein said gate electrode forms a Schottky barrier with said active region and said source and drain electrodes form ohmic contacts with said active region.
 9. The semiconductor device assembly of claim 1 wherein said wafer is GaAs. 